The present invention relates to magnetically responsive devices such as magnetic mines, more particularly to methods and apparatuses for evaluating the performance of a ship's degaussing system with respect to threats posed by magnetic mines that are situated in a marine environment.
A mine is an explosive device which is usually concealed either underground or underwater, and which is used primarily by military forces for defensive purposes. Mines typically are self-contained devices which include an explosive capability and a detonator (a firing mechanism for triggering the mine explosion), and which explode when touched by or approached by a target. “Minefields” are areas where mines have been placed. Generally there are two categories of mines, based on their situation, viz., “land mines” and “underwater mines” (synonymously referred to as “water mines,” “submarine mines,” “sea mines” or “naval mines”).
An underwater mine is a mine which is situated in or on water or contiguously with respect to water or which otherwise bears physical or functional relation to a water environment. A typical underwater mine comprises an explosive charge positioned underwater and set to fire in response to the presence of a marine vehicle (e.g., a ship or submarine) in contact therewith or in proximity thereto. Underwater mines are generally laid in the water for purposes of damaging or sinking ships or of deterring ships from entering an area. “Moored mines” are underwater mines having positive buoyancy, typically held below the water surface at a pre-selected depth by a mooring (e.g., cable) attached (e.g., tethered) to an anchor (e.g., on a sea bottom). “Bottom mines” are underwater mines having negative buoyancy and resting on a seabed (e.g., at the bottom of relatively shallow water). “Floating mines” are underwater mines that are not entirely underwater but are visible on the surface.
Underwater mines are triggered either by direct contact or by indirect influence. Typically, when an underwater mine is triggered, an expanding gas sphere caused by the explosion sends shock waves through the water, these shock waves having deleterious effects on the nearby target marine vessel. “Contact mines” are actuated as a result of physical contact between the target ship and the mine's casing or one or more of the mine's appendages (e.g., rods or antennae protruding from the mine's surface). “Influence mines” are actuated either as a result of sensing an “influence field” emanating from the target marine vessel, or as a result of the target marine vessel's intrusion within an “influence field” emanating from the mine. Generally, influence mines sense changes in physical patterns in surrounding water, such as pertaining to magnetic fields (“magnetic mines”), pressure change (“pressure mines”) or sound waves (“acoustic mines”).
U.S. Navy surface combatant ships are equipped with degaussing systems comprising a set of current-carrying coils which are adjusted to reduce the ship's magnetic field and thereby reduce it's vulnerability to the magnetic mine threat. Currently, performance of U.S. naval combatant degaussing systems is determined by recording the combatant's magnetic field at a Magnetic Silencing Facility (MSR), measuring the peak field, and adjusting degaussing coil currents to reduce this peak field to less than a specified level.
However, magnetic mines do not operate by measuring the peak value of a ship's magnetic field; rather, magnetic mines operate by measuring the rate of change of a ship's magnetic field. In addition, many mines measure the rate of change in the ship's horizontal magnetic fields to determine when to actuate. Current methods for measuring combatant degaussing system performance may not reflect the combatant's actual susceptibility to the magnetic bottom mine threat.